Summary:
The changes in https://github.com/facebook/rocksdb/commit/8272a6de57ed701fb25bb660e074cab703ed3fe7 were untested with `USE_HDFS=1`. There were a couple compiler errors. This PR fixes them.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5444

Test Plan:
```
$ EXTRA_LDFLAGS="-L/tmp/hadoop-3.1.2/lib/native/" EXTRA_CXXFLAGS="-I/tmp/hadoop-3.1.2/include" USE_HDFS=1 make -j12 check
```

Differential Revision: D15885009

fbshipit-source-id: 2a0a63739e0b9a2819b461ad63ce1292c4833fe2